1) Field of the Invention
The invention relates to a process for preparing aqueous polymer dispersions, and also to their use for preparing water-redispersible polymer powders.
2) Background Art
Protective-colloid-stabilized aqueous polymer dispersions, for example for construction sector applications, are known. To ensure that the aqueous polymer dispersions are stable, protective colloids, such as polyvinyl alcohol, polyvinylpyrrolidone, cellulose or starch, are used during the polymerization in an aqueous medium. EP-A 133899 (U.S. Pat. No. 4,532,295) has disclosed starch-stabilized polymer dispersions. WO-A 84/00369 (U.S. Pat. No. 4,528,315) and EP-B 62106 (U.S. Pat. No. 4,397,968) describe dispersions stabilized using polyvinyl alcohol.
However, the addition of significant amounts of protective colloids, which are necessary to ensure sufficient stability, is attended by disadvantages: protective colloids are relatively expensive, their addition reduces the binder content of the dispersion, rheological disadvantages result and, finally, the addition of protective colloids makes the polymer films produced from the dispersion susceptible to attack by water. The susceptibility to attack by water is, in particular, undesirable in many applications in the construction sector and coatings sector, and has to be counteracted by subsequent steps during formulation. Reactivity with water is particularly disadvantageous in highly polymer-filled cement applications, such as sealing slurries.
To improve the water resistance of polymer films which have been prepared using protective-colloid-stabilized polymer dispersions EP-A 727441 recommends the use of water-soluble protective colloids based on from 5 to 50% of comonomers with acid and/or anhydride functions, from 0.1 to 80% of long-chain (meth)acrylates and up to 94.9% of water-insoluble principle monomers, such as (meth)acrylates or vinyl esters. A disadvantage is that these protective colloids are prepared by polymerization in organic solvents, and complicated solvent removal by distillation is required.
DE-A 2618898 (U.S. Pat. No. 3,965,032) describes protective colloids based on copolymer polyelectrolytes having nonionic hydrophobic units, such as methyl methacrylate, and hydrophilic units, such as ethylenically unsaturated comonomers substituted with sulfonate groups and prepared by polymerization in organic solvents. These protective colloids have disadvantages which are the same as the abovementioned EP-A 727441.
DE-A 19608911, which is a subsequent publication, discloses crosslinkable protective colloids having sulfonate-containing, N-methylol-containing and hydrophobic monomer units. A disadvantage is that when polymers stabilized with protective colloids of this type are used in acid media or exposed to heat they crosslink and therefore become brittle. This is contrary to the properties of flexibility required in many applications (sealing slurries, sealing compositions, and paints for bridging cracks).
EP-B 206814 describes the use of terpolymers made from (meth)acrylic acid and from sulfonate-functional monomer and vinyl esters for stabilizing aqueous systems which comprise dissolved or suspended solids, for example cooling water or boiler water.
EP-A 671435 discloses that copolymers made from alkyl acrylates and from sulfonate- or carboxylate-substituted monomers are suitable spraying aids for spraying protective-colloid-stabilized dispersions. EP-A 629650 (U.S. Pat. No. 5,462,978) describes copolymers made from sulfonic-acid-functional comonomers and from water-insoluble comonomers as spraying aids in the spray drying of aqueous polymer dispersions.
The object on which the invention was based was to provide protective-colloid-stabilized aqueous polymer dispersions which have better water resistance when used in the construction sector or coatings sector than previously known polymer dispersions, for example those stabilized with polyvinyl alcohol.
The invention provides a process for preparing protective-colloid-stabilized aqueous polymer dispersions by free-radical polymerization of ethylenically unsaturated monomers by emulsion polymerization, which comprises carrying out the polymerization in the presence of one or more protective colloids comprising
a) from 5 to 95% by weight of monomer units containing sulfonic acid groups or sulfonate groups,
b) from 5 to 95% by weight of noncrosslinkable, water-soluble monomer units, and
c) from 0 to 5% by weight of hydrophobic monomer units selected from the class consisting of water-insoluble ethylenically unsaturated compounds, where the proportions in % by weight are based on the total weight of the copolymer.
Suitable monomer units a) are water-soluble, free-radical-polymerizable, ethylenically unsaturated compounds which contain sulfonic acid groups and, respectively, sulfonate groups xe2x80x94SO3M, where Mxe2x95x90H, an alkali metal ion, an ammonium ion or an alkaline earth metal ion. Preference is given to 2-acrylamido-2-methylpropanesulfonic acid (AMPS), styrenesulfonic acid, sulfoalkyl (meth)acrylates, sulfoalkyl itaconates, in each case preferably having a C1-C6-alkyl radical, vinylsulfonic acid and ammonium, alkali metal or alkaline earth metal salts thereof. Particular preference is given to 2-acrylamido-2-methylpropane-sulfonic acid (AMPS), styrenesulfonic acid, sulfopropyl acrylate, sulfopropyl itaconate, vinylsulfonic acid and ammonium, sodium, potassium and calcium salts thereof.
Preferred monomer units b) are water-soluble, free-radical-polymerizable, ethylenically unsaturated compounds which contain carboxyl groups xe2x80x94COOM, where Mxe2x95x90H, an alkali metal ion, ammonium ion or alkaline earth metal ion, or contain amide groups xe2x80x94CONH2 or contain hydroxyl groups. Particular preference is given to acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, hydroxyalkyl (meth)acrylates, such as hydroxyethyl acrylate or hydroxypropyl acrylate, hydroxybutyl acrylate, acrylamide and methacrylamide. Acrylamide and methacrylamide are most preferred.
For the purposes of the present invention, water-soluble generally implies solubility in water of at least 10% by weight at 23xc2x0 C.
Suitable monomer units c) are free-radical-(co)polymerizable, ethylenically unsaturated compounds which have less than 4% by weight water-solubility at 23xc2x0 C. Preference is given to esters of acrylic or methacrylic acid with alcohols having from 1 to 18 carbon atoms, such as methyl methacrylate, methyl acrylate, N-butyl acrylate, 2-ethylhexyl acrylate; vinylaromatics, such as styrene or vinyltoluene; olefins, such as ethylene, propylene or butadiene; vinyl halides, such as vinyl chloride; vinyl esters of aliphatic carboxylic acids having from 1 to 18 carbon atoms, such as vinyl acetate, vinyl propionate, isopropenyl acetate, vinyl laurate, and vinyl esters of xcex1-branched monocarboxylic acids having from 5 to 11 carbon atoms, such as VeoVa9(copyright) or VeoVa10(copyright), Shell products which are vinyl esters of verstatic acid, or a saturated monocarboxylic acid mixture of highly branched C-9 and C-10 isomers.
Preferred protective colloids have
a) from 20 to 95% by weight of monomer units containing sulfonic acid groups and/or sulfonate groups and
b) from 5 to 80% by weight of monomer units containing carboxyl groups, amide groups or hydroxyalkyl groups, in particular protective colloids having
a) from 20 to 95% by weight of monomer units containing sulfonic acid groups or sulfonate groups and
b) from 5 to 80% by weight of monomer units containing amide groups, where the proportions in % by weight are in each case based on the total weight of the copolymer.
Particularly preferred protective colloids have
a) from 40 to 60% by weight of monomer units which derive from one or more monomers selected from the class consisting of 2-acrylamido-2-methylpropane-sulfonic acid, styrenesulfonic acid, sulfoalkyl (meth)acrylates, sulfoalkyl itaconates, in each case preferably with a C1-C6-alkyl radical, vinylsulfonic acid and salts thereof, and
b) from 40 to 60% by weight of monomer units which derive from acrylamide and/or from methacrylamide.
The protective colloids are preferably prepared by free-radical polymerization in aqueous solution at a reaction temperature of preferably from 40 to 80xc2x0 C. Initiation is by the usual water-soluble free-radical generators, preferably used in amounts of from 0.01 to 3.0% by weight, based on the total weight of the monomers. Examples of these are ammonium persulfate, potassium persulfate, hydrogen peroxide, potassium peroxodiphosphate, sodium peroxodiphosphate and ammonium peroxodiphosphate. If desired, the free-radical initiators mentioned may also, in a known manner, be combined with from 0.01 to 1.0% by weight, based on the total weight of the monomers, of reducing agents. Examples of suitable reducing agents are alkali metal formaldehyde sulfoxylates and ascorbic acid. To adjust the molecular weight, the regulators usually used may be added during the polymerization, for example mercaptans, aldehydes and chlorinated hydrocarbons. The copolymers are preferably used in the form of their aqueous solutions. Solids contents are preferably adjusted to from 15 to 25% by weight, depending on the applications.
The novel process is suitable for free-radical polymerization of ethylenically unsaturated monomers, for example of vinyl esters of unbranched or branched carboxylic acids having from 1 to 18 carbon atoms, of esters of acrylic or methacrylic acid with unbranched or branched alcohols having from 1 to 18 carbon atoms, of vinylaromatics, of vinyl halides and of olefins.
Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate and vinyl esters of xcex1-branched monocarboxylic acids having 5 or 9 to 11 carbon atoms, for example VV5R, VeoVa9R or VeoVa10R. Vinyl acetate is particularly preferred.
Preferred methacrylates or acrylates are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, 2-ethylhexyl acrylate and 2-ethylhexyl methacrylate. Particular preference is given to methyl acrylate, methyl methacrylate, n-butyl acrylate and 2-ethylhexyl acrylate.
Preferred vinylaromatics are styrene, xcex1-methylstyrene, o-chlorostyrene or vinyltoluenes. Preferred vinyl halides are vinyl chloride and vinylidene chloride. Preferred olefins are ethylene, propylene, 1,3-butadiene and isoprene.
If desired, from 0.05 to 30.0% by weight, preferably from 0.5 to 15% by weight, based in each case on the total weight of the monomers, of one or more comonomers may also be added, for example for crosslinking or to modify the adhesion properties of the polymers prepared according to the invention. The amounts of crosslinking monomers used are preferably from 0.5 to 5.0% by weight, based on the total weight of the monomers. Examples of these are N-methylolacrylamide, N-(alkoxymethyl)acrylamides or N-(alkoxymethyl)methacrylamides having a C1-C6-alkyl radical, such as N-(isobutoxymethyl)acrylamide (IBMA), N-(n-butoxymethyl)acrylamide (NBMA); comonomers with more than one ethylenic unsaturation, such as ethylene glycol diacrylate, divinyl adipate, divinylbenzene, diallyl phthalate or triallyl cyanurate. Examples of comonomer units suitable for modifying adhesion properties are hydroxyalkyl methacrylates, and hydroxyalkyl acrylates, such as hydroxyethyl, hydroxylpropyl or hydroxylbutyl acrylate or methacrylate, and also compounds such as diacetoneacrylamide and acetylacetoxyethyl acrylate or methacrylate.
Using the novel method, the aqueous polymer dispersions are prepared by aqueous emulsion polymerization in the presence of from 1 to 20% by weight, preferably from 5 to 10% by weight, of one or more of the protective colloids mentioned, based in each case on the total weight of the monomers.
The polymerization temperature is generally from 35 to 95xc2x0 C., preferably from 40 to 80xc2x0 C. The polymerization may be carried out as a batch process, where all of the components are in the initial reactor charge, or by the feed process, where one or more components are added during the polymerization. Variants of mixing which have an initial charge and a feed are preferred. The feeds may be separate (spatially and chronologically) or some or all of the components to be fed may be preemulsified before being fed.
The protective colloids may be within the initial charge or in the feed, or divided between initial charge and feed. The protective colloids are preferably within the initial charge in the form of their aqueous solutions.
If, for example, gaseous reaction components are used, the emulsion polymerization may also be carried out at elevated pressure. For operations under elevated pressure, for example when using the monomers vinyl chloride or ethylene, pressures of from 5 bar to 100 bar are preferred. An example of a factor in the decision is the desired amount of ethylene to be polymerized.
The protective colloids toube used according to the invention may be used either alone or combined with emulsifiers. Accompanying emulsifiers may be anionic or nonionic emulsifiers. If the polymerization is carried out in the presence of emulsifiers, the amount of these is preferably up to 4% by weight, based on the total weight of the monomer phase. Preference is given to the use of anionic or nonionic emulsifiers. Examples of commonly used emulsifiers are ethoxylated fatty alcohols having a C8-C36-alkyl radical and an ethoxylation number (EO number) of from 3 to 50; ethoxylated mono-, di- and trialkylphenols having a C4-C10-alkyl radical and an EO number of from 3 to 50; alkali metal salts of di-C4-C12-alkyl esters of sulfosuccinic acid. Other suitable compounds are alkali metal salts and ammonium salts of C8-C12-alkyl sulfates, of ethoxylated alkanols having a C12-C18-alkyl radical and an EO number of from 3 to 30, of ethoxylated C4-C10-alkylphenols having an EO number of from 3 to 50, of C12-C18-alkylsulfonic acids, of C9-C18-alkylarylsulfonic acids and of sulfonates of ethoxylated, linear or branched C8-C36-alkylalcohols having an EO number of from 3 to 50. In the most preferred embodiment the polymerization is carried out without adding emulsifier.
The polymerization is initiated with the water-soluble, thermal initiators or redox-initiator combinations commonly used for emulsion polymerization. Examples of thermal initiators are organic peroxides, such as tert-butyl hydroperoxide and cumyl hydroperoxide, or persulfates, such as potassium persulfate, or H2O2, or azo compounds, such as azodiisobutyronitrile. Redox initiators preferably used are hydrogen peroxide, tert-butyl hydroperoxide or potassium persulfate combined with hydroxymethanesulfinic acid, ascorbic acid or sodium sulfite as reducing agent. The amount of initiator is preferably from 0.01 to 1.0% by weight, based on the total weight of the monomer phase.
To control the molecular weight, regulating substances may be used during the polymerization. The amounts of these used are usually from 0.01 to 5.0% by weight, based on the monomers to be polymerized, and they are fed separately or else premixed with reaction components. Examples of substances of this type are dodecylmercaptan, mercaptopropionic acid, methyl mercaptopropionate, isopropanol and acetaldehyde.
The aqueous dispersions obtainable by the novel process have a solids content of from 30 to 75% by weight, preferably from 40 to 65% by weight.
The protective-colloid-stabilized, aqueous polymer dispersions are also suitable for producing water-redispersible polymer powders. For this, the aqueous dispersions are dried. The dispersions are preferably spray-dried or freeze-dried. The dispersions are most preferably spray-dried.
As spraying aids, to ensure redispersibility it is necessary to add further water-soluble protective colloids to the dispersion prior to drying. These are different from the sulfonic-acid-functional or, respectively, sulfonate-functional protective colloids used for the polymerization. Examples of substances of this type which are widely commercially available are: polyvinyl alcohols, polyvinylpyrrolidone, cellulose derivatives, starch derivatives, and water-soluble condensation products made from melamine and formaldehyde or from naphthalenesulfonic acid and formaldehyde. Preference is given to polyvinyl alcohols. The amount of the spraying aid generally used is from 5 to 25% by weight, based on the polymeric constituents of the dispersion.
A content of up to 1.5% by weight of antifoam, based on the base polymer, has frequently proven advantageous during spraying. To increase storage capability by improving resistance to blocking, in particular for powders with a low glass transition temperature, the powder obtained can be admixed with an antiblocking agent (anticaking agent), preferably up to 30% by weight, based on the total weight of polymeric constituents.
Surprisingly, and despite their redispersibility, the dispersion powders obtained in this way give, for example in modifying hydraulically setting compositions of building materials, products with better water resistance than is given by conventional dispersion powders.
The protective-colloid-stabilized polymer dispersions and the dispersion powders obtainable therefrom are suitable as binders for coatings and renders, in particular paints; as adhesives or binders for wood, paper, textiles or nonwovens; as binders in papermaking and for producing molding compositions and moldings; as binders for use in the construction industry, in particular as additives to concrete, to construction adhesives, to mortars, to troweling compositions and to leveling compositions.
The examples below further illustrate the invention.
Preparation of the Protective Colloids